Defects in neuronal migration and axonal guidance underlie many inherited and sporadic disorders, yet the molecular mechanisms that control the final position of neurons and their wiring remain elusive. Secreted or substrate-bound molecules that act as attractants or repellents control both neuronal migration and axonal guidance during CNS development. Several guidance cues, including the secreted netrins, have been shown to be bifunctional; that is, they have both attractive and repulsive properties. We positionally cloned the rostral cerebellar malformation (rcm) gene and have demonstrated that it encodes a vertebrate homolog of UNC-5, a C. elegans transmembrane receptor necessary for cell and axon migrations away from netrin sources. We have demonstrated that this netrin receptor, now named UNC5H3, is necessary for the exclusion of migrating cerebellar granule cells from the developing midbrain and brainstem, and guidance of corticospinal tract axons. Our recent data reveals that UNC5H3 is also necessary for motor axon guidance in the peripheral nervous system, in a genetic background dependent fashion. This application proposes to examine the function of Unc5h3 and other netrin receptors in PNS development. Further, as the first steps in identifying loci that genetically interact with Unc5h3, we propose to genetically identify modifier loci that suppress Unc5h3- mediated motor axon defects. We also will investigate the role of other vertebrate UNC5 family members in the development of the midbrain and hindbrain. Lastly, we outline experiments to begin dissecting UNC5H3 signaling pathways during CNS development. The results of these experiments will have a significant impact on our understanding of the function of these guidance receptors during CNS development.